Medical devices are used to treat patients suffering from a variety of conditions. Examples include implantable medical devices (IMDs) such as implantable pacemakers and ICDs, which are electronic medical devices monitoring the electrical activity of the heart and, when necessary, providing therapeutic electrical stimulation to one or more of the heart chambers. For example, a pacemaker senses an arrhythmia episode (i.e., a disturbance in heart rhythm) and provides appropriate therapeutic electrical stimulation pulses, at a controlled rate, to selected chambers of the heart in order to terminate the arrhythmia and restore the proper heart rhythm. The types of arrhythmias detected and corrected by pacemakers include bradycardia, which are unusually slow heart rates, and tachycardia, which are unusually fast heart rates.
Medical devices such as ICDs also detect arrhythmias and provide appropriate electrical stimulation pulses to selected chambers of the heart to correct the abnormal heart rate and/or rhythm. In contrast to pacemakers, however, an ICD can also deliver much stronger and less frequent pulses of therapeutic electrical stimulation (e.g., cardioversion and/or defibrillation therapy). This is because ICDs are generally designed to terminate episodes of cardiac fibrillation (e.g., episodes of rapid, unsynchronized quivering of one or more heart chambers) and severe tachycardia (e.g., very rapid but relatively coordinated contractions of the heart). To correct such arrhythmias, an ICD delivers a low, moderate, and/or high-energy electrical therapy to the heart.
The typical defibrillator or cardioverter includes a set of electrical leads, which extend from a sealed housing into operative contact with a portion of a heart. Within the housing are a battery for supplying power, one or more capacitors coupled to the battery and adapted to rapidly deliver bursts of electric energy via the leads to the heart, and monitoring circuitry for monitoring cardiac activity and determining when, where, and what electrical therapy to withhold or apply. The monitoring circuitry generally includes a microprocessor and a computer readable memory medium storing instructions not only dictating how the microprocessor answers therapy questions, but also controlling certain device maintenance functions, such as maintenance of the capacitors in the device.
With respect to ICDs, typically at least two capacitors are electrically coupled to the heart. One type of capacitor adapted for use in conjunction with an ICD includes aluminum electrolytic capacitors, although other types have been used as well. This type of capacitor usually includes sheets of aluminum foil and electrolyte-impregnated separator material. Each strip of aluminum foil is covered with an aluminum oxide, which insulates the foils from the electrolyte in the paper. One maintenance issue with aluminum electrolytic capacitors concerns the degradation of their charging efficiency after long periods of inactivity. The degraded charging efficiency, which stems from instability of the aluminum oxide in the liquid electrolyte, ultimately requires the battery to progressively expend more and more energy to charge the capacitors prior to delivering cardioversion or defibrillation therapy.
Thus, to repair this oxide degradation, microprocessors are typically programmed to reform the degraded (or deformed) aluminum oxide. The capacitor reform process typically involves at least one capacitor charge-discharge cycle. For example, an aluminum capacitor is typically rapidly charged and held at or near a rated or maximum-energy voltage (the voltage corresponding to maximum energy) for a time period (e.g., on the order of less than about one minute), before being discharged internally through a non-therapeutic load. In some cases, the maximum-energy voltage is allowed to leak off slowly rather than being maintained; in others, it is allowed to leak off (or droop) for 60 seconds and discharged through a non-therapeutic load; and in still other cases, the voltage is alternately held for five seconds and drooped for 10 seconds over a total period of 30 seconds, before being discharged through a non-therapeutic load. These periodic charge-hold-discharge (or charge-hold-droop-discharge) cycles for capacitor maintenance are referred to as “reforming.” Unfortunately, reforming aluminum electrolytic capacitors tends to reduce battery life.
The aluminum electrolytic capacitors used in early ICDs exhibited relatively low energy density (<2 J/cc) and therefore contributed a large amount to the overall device volume. To decrease capacitor and ICD volume, medical device designers, suppliers and manufacturers developed implantable wet-tantalum capacitors. In addition to having a higher energy density (>4 J/cc) such capacitors exhibit a slightly lower rate of de-formation, and therefore require less energy to effectively reform the oxide layer of the capacitor. More recently, wet-tantalum capacitors that require very little or no reformation have been developed. To wit, non-provisional U.S. patent application Ser. No. 10/448,594 filed 30 May 2003 and entitled, “WET TANTALUM CAPACITOR USABLE WITHOUT REFORMATION AND MEDICAL DEVICES FOR USE THEREWITH” and non-provisional U.S. patent application Ser. No. 10/431,356 filed 7 May 2003 entitled, “WET TANTALUM REFORMATION METHOD AND APPARATUS” are directed to such subject matters, and the contents of each are hereby entirely incorporated by reference herein.
While substantially eliminating the need for capacitor reformation, these low or non-reformation capacitors may contribute to energy-management issues within an ICD. For example, capacitors not requiring reformation can cause the ICD battery to operate without a high current pulse for a long period of time (e.g., months to years). Most implantable device batteries are comprised of Li/SVO (lithium/silver vanadium oxide), a lithium anode, and a silver vanadium oxide cathode. When there is an extended period between high voltage therapies or other high current events, the battery can develop a deleterious resistive film on the surfaces of an anode disposed within the battery. First, the lithium anode forms its own passive film by reaction with electrolyte. This film, commonly referred to as the solid electrolyte interface (SEI), is typically harmless since it comprises an electrically conductive film. However, the SEI film can increase in thickness over time, and thus contribute increased electrical resistance to the operative circuitry of an ICD.
Second, the SVO material also forms a film on the lithium surface. This is equally undesirable as it results in further increases in film thickness and in electrical resistance over time. Generally, these problems are resolved during the reformation process or as a result of one or more high current pulses, during which the resistive film will dissipate (or “slough off”) thus providing a fresh lithium surface.
However, if an IMD uses a capacitor that does not require reformation, then over time the above-mentioned SEI films can be expected to only increase in size and electrical resistance. If this occurs, subsequent high-current pulses will suffer from “voltage delay” due to the increased resistance of the films. Voltage delay occurs when the highly resistive SEI film causes the voltage delivered during a high current pulse to be lower than a desired magnitude (e.g., less energy than that which would be delivered in the absence of resistive SEI film). This voltage delay will occur until the film sloughs off the lithium. Voltage delay is undesirable in that it causes the battery to take longer to charge the capacitor and reduces battery life. For example, if the battery voltage is lower during the high current pulse, it is delivering less energy per unit time. Therefore, it takes more charge out of the battery to provide the pulse and it takes longer to charge the capacitor. This problem is exacerbated the longer the battery goes without a capacitor reformation or a high current pulse.
Another problem is created if the battery voltage is too low. In most ICDs there is a minimum voltage or a voltage floor representing the voltage necessary to continue to power the ICD circuitry while the capacitor is being charged. Generally, if the battery voltage drops below this voltage floor, a power-on-reset (POR) can occur where the ICD will suspend any therapy currently in progress. To combat this problem ICDs will generally implement a safety feature, which prevents the battery voltage from dropping below the voltage floor by lowering the current drawn from the battery when the battery voltage approaches the voltage floor. However, by drawing less current from the battery the process of charging the capacitor is lengthened. This is undesirable as it is commonly accepted that the odds of survival or recovery from a potentially lethal arrthymia such as ventricular fibrillation (VF) decrease significantly as the amount of time taken to deliver a cardiac therapy to terminate an episode of VF increases. A voltage delay may exacerbate this problem in two ways. First, the voltage delay would increase the time required to charge the capacitor by reducing available battery power. Second, a voltage delay may cause the battery voltage to drop below the voltage floor thus initiating the POR safety feature, further reducing available battery power and increasing capacitor charge time. This could potentially prevent an appropriate therapy from even being administered as some devices will either cease charging, or deliver a reduced energy after a predetermined charge interval.
Yet another potential adverse impact of voltage delay is reduced device longevity. Typically ICDs are designed to declare the Elective Replacement Indicator (ERI) when the background voltage of the battery (voltage in the absence of capacitor charging) reaches a predetermined level. However, most devices incorporate a secondary mechanism for declaring ERI when the capacitor charge time reaches a predetermined, excessively high level. In the event that voltage delay results in such an excessively long charge time, these devices will trigger the ERI well before battery depletion, significantly reducing the longevity of the ICD. Therefore, a voltage delay not only delays the delivery of a cardiac therapy it also has an impact on the overall device operation.
In prior ICDs the energy required to reform the capacitor was much greater than that required to remove the anode film from the battery, and therefore dominated any increase in capacitor charge time. Furthermore, the need to frequently reform the capacitor provided the required periodic conditioning of the battery necessary to minimize the effects of voltage-delay. However, now that capacitors needing little to no reformation have been developed, the potential exists for voltage-delay to become substantial, resulting in extended capacitor charge times when long periods of time pass between capacitor charging or other high current drain events driven by the battery. Changing trends within the industry may further exacerbate this. As ICDs have evolved, detection algorithms and therapies have become more sophisticated. As a result, devices are more frequently treating potentially fatal arrhythmias with alternative low-energy therapies (e.g., so-called anti-tachycardia pacing or “ATP”), thereby dramatically reducing the number of high-energy therapies being administered. In addition, indications for ICD implantation have been expanded to include patients who are expected to require only very few high-energy cardioversion or defibrillation therapies over the life of the device.